long_running_job.kt

private val aMutex = Mutex()
private var job: Job? = null

private fun getJob(from: Int) = launch {
    try {
        var counter = from
        while (isActive) withContext(NonCancellable) {
            println(counter++)
            delay(100)
        }
    } catch (e: CancellationException) {
        println("end") // graceful shutdown
    }
}

suspend fun startWork(from: Int) = aMutex.withLock {
    if (job?.isCancelled == false) throw RuntimeException("work already started")

    job = getJob(from)
}

suspend fun stopWork() = aMutex.withLock {
    if (job == null) throw RuntimeException("work already stoped")

    job?.cancel()
    job?.join()
}